Electrospray as a Controlled-Current Electrolytic Cell: Electrochemical Ionization of Neutral Analytes for Detection by Electrospray Mass Spectrometry

Abstract

In this paper an electrospray ion source is shown to be a controlled-current electrolytic flow cell which, when operated so that three key requirements are met, can be used for efficient neutral analyte ionization (i.e., complete analyte electrolysis) and sensitive gas-phase detection (i.e., minimized gas-phase signal suppression) in electrospray mass spectrometry (ES-MS). The means to meet these requirements are discussed, including the addition of an appropriate electrolyte to the electrosprayed solutions (e.g., lithium triflate), the use of slower flow rates (e.g., 5.0 vs 40 {mu}L/min), and the use of a platinum capillary in the ES device, rather than the more commonly used stainless steel capillary. Neutral metallocenes, metalloporphyrins, and polycyclic aromatic hydrocarbons are used as the model compounds. Operation of the ES ion source in the manner described expands the neutral compound types amenable to low level detection by ES-MS to include even those that are relatively difficult to oxidize (i.e., E > 1.0 V vs SCE) and, therefore, also expands the universality of ES as an ionization source. From the electrochemical point of view, this operation of the ES ion source might be viewed as a means to provide molecular weight information, and possibly the structure, for the ionicmore » products formed during a controlled-current electrolysis experiment. 32 refs., 5 figs.« less